1. Observatoire Astronomique de Strasbourg, Université Louis Pasteur, 11, rue de I'Université, 67000 Strasbourg, France
2. Departments of Astronomy and Physics and the Center for Particle Astrophysics, University of California, Berkeley, CA 94720, USA
3. NASA/Fermilab Astrophysics Center, Fermi National Accelerator Laboratory, Batavia, IL 60510-0500, and the Departments of Physics and of Astronomy and Astrophysics, Enrico Fermi Institute, University of Chicago, Chicago, IL 60637-1433, USA